The invention relates to scannable-beam microscopes and image stores therefor and more specifically to scanning electron microscope arrangements and to image store arrangements therefor.
In known forms of scanning electron microscopes, an electron beam is directed at the surface of a specimen and causes secondary electrons to be produced from the specimen which are collected to produce an electrical signal which controls the brightness of a cathode ray tube. A scanning unit is provided which energises scanning coils with a suitable waveform so as to cause the electron beam to perform a scan, normally a frame scan, of the surface of the specimen, and this scanning unit is also arranged to control the time base of the CRT so that the CRT beam scans the CRT screen synchronously with the scanning of the specimen. In this way, an image of the specimen surface, in terms of the emitted secondary electrons, is produced.
A disadvantage of such an arrangement is that because the electron beam in the microscope column is scanned over the specimen surface synchronously with the beam in the CRT, both the column beam and the CRT beam must be capable of being scanned at the same rate. This means that the scanning coils and associated amplifying circuitry for scanning the column beam have to be capable of operating over a wide speed range, say from normal television rate at 50 or 60 Hz down to 100 seconds for a complete frame scan, this range of scanning speeds being necessary in order to produce signals of the required signal-to-noise ratio from a variety of different specimens. The wide range of required scanning speeds complicates the design of scanning coils and associated circuitry and imposes high power requirements on these components. In addition, such an arrangement complicates the CRT, which again has to be capable of handling a variety of scanning rates and will therefore need to have a high persistence screen.
It is also known to provide such arrangements with image stores for temporarily storing the signals produced by the emitted secondary electrons. However, the scanning coils and associated circuitry for scanning the column beam are still designed to cope with a wide range of scanning rates and therefore complex in design, and the CRT is also still arranged to handle a variety of scanning rates.